Two-point modulators normally have a phase locked loop (PLL), to which a modulation signal for the phase and/or frequency modulation of a carrier signal is applied at two points. In this case, one point having a high-pass filter behavior and one point having a low-pass filter behavior are usually provided within the phase locked loop, the modulation signal being introduced at each of said points. A superposed signal results at the output of the phase locked loop, with the advantage that the loop exhibits a frequency-independent transmission response.
Two-point modulator arrangements are mentioned, for example, in the document DE 199 29 167 A1. Provided there between the loop filter and the control input of a VCO is a summation point, at which the modulation signal is combined with the output signal of the loop filter. In this case, a high-pass filter transmission response is formed for the modulation frequency. A frequency divider is provided in the feedback path of the phase locked loop, the divider ratio of which is likewise fed in by the modulation signal so as to form a low-pass filter transmission response for the modulation frequency.
Overall, the two-point modulation principle enables modulation, by means of a phase locked loop, with modulation signals whose bandwidth is wider than the bandwidth of the phase locked loop itself.
It has hitherto been a prerequisite for the known two-point modulator that the voltage-controlled oscillator comprises two control inputs, namely one that is driven by the output of the phase comparator via the loop filter and a second that is driven by the modulation signal.
Alternatively, an additional combination element for combining the modulation signal with the output signal of the loop filter could be provided in the phase locked loop but this signifies additional outlay and significantly impairs the noise properties of the modulator.